The present invention relates generally to prosthetic joints, and more particularly to a shoulder prosthesis. The invention has specific application with respect to the humeral component of the shoulder prosthesis.
Conventional prostheses for the replacement of the shoulder joint include a segment engaged within the humerus bone and a mating articulating segment affiliated with the glenoid bone. In the typical shoulder prosthesis, the upper portion of the humerus is replaced by a unitary structure. This structure includes a stem designed to extend downwardly into bore or cavity formed within the humerus. This stem is secured within the bone by bone cement or through the use of coatings designed to promote ingrowth to secure the stem in place. Again with the conventional prosthesis, the stem is attached to a body portion that is designed to replace portions of the humerus at the anatomical neck of the bone. A generally spherical head portion projects from a surface of the body. This spherical head mates with a complementary concave articulating component mounted within the glenoid.
In recent years, modular shoulder prostheses have been developed to account for the different anatomies of the shoulder joint among patients. For instance, differently sized prostheses are necessary to accommodate the different bone sizes of prospective patients. Similarly, different shoulder joints require different angles of inclination of the articulating elements relative to the long axis of the humerus bone. Thus, a variety of modular prostheses have been developed that permit substitution of particular components of the prosthesis as necessary prior to implantation. One example of such a shoulder prosthesis system is disclosed in U.S. Pat. No. 5,314,479, owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference.
One problem faced by both the conventional and the modular shoulder prosthesis is the deterioration of the shoulder joint that can accompany a shoulder arthroplasty. For instance, a patient who has undergone shoulder arthroplasty may experience loss of soft tissue strength, specifically at the rotator cuff, which can eventually lead to a total loss of key constraints that maintain the patency of the joint. This loss of soft tissue and soft tissue strength can allow unnatural joint loads to be produced, which can compromise the function of the prosthetic joint and/or lead to joint pain.
One solution for this problem for this problem is revision of the shoulder prosthesis. This revision can entail the substitution of different articulating components, or differently sized prosthetic components. In one treatment, the shoulder prosthesis is changed to a “reverse” type of prosthesis. A typical prosthetic shoulder replicates the standard anatomy of the joint. Specifically, the humeral component provides a convex articulating surface, much like the natural humeral end of the bone. This convex surface mates with a concave glenoid component.
A “reverse” type prosthesis essentially reverses the arrangement of the articulating surfaces. Specifically, the glenoid component includes a convex or partially spherical component, while the complementary concave surface is integrated into the humeral component. One consideration involved in the use of a reverse prosthesis is that the concave articular surface that is now part of the humeral component may actually protrude into the metaphyseal region of the humerus. This modified geometry can require modification of the metaphyseal portion of the humerus bone as well as the prosthesis.
Prior systems have required total revision of the joint. A total revision entails removal of the entire implant, including the stem that is fixed within the diaphyseal portion of the humerus. Of course, this surgical procedure is very difficult and invasive, and can place the patency of the patient's existing bone at risk. In view of these deleterious effects, what is needed is a shoulder prosthesis system that simplifies the revision process and that allows ready replacement of a standard joint component with a reverse-type component.